If you look at water molecule H2O, it has an electro-negative atom oxygen and electropositive atom hydrogen. Due to the electronegativity difference, oxygen always has a partial negative charge and hydrogen has a partial positive charge. Intermolecular hydrogen bonding exists between these polar water molecules. Hydrogen bonding is between hydrogen atom of one water molecule and oxygen atom of another water molecule (H2O - - - - H2O). Though its a weak force of attraction, energy is needed to break these bonds. Collectively this energy is considerable and so the boiling point of H2O is 100 degree Celsius.
When water boils, inter-molecular hydrogen bonds break and result in change of phase from liquid to vapour phase.
